1. Field of the Invention
This invention relates to a variable magnification optical system, particularly used in copying machines and others, having a compact construction in which, upon changing the magnification, it can be moved as one body while predetermined lenses in the lens system can be moved with a small movement distance to reduce aberrations.
2. Description of the Prior Art
In the prior art copying machines and others wherein a fixed focus lens is used as a projector system, it has been required that, upon changing of the magnification, the mirror was moved with the lens for compensating the optical path length dependent on the changed magnification. The lens is generally separated spatially from the mirror for compensating the optical path length resulting in a difficulty on the continuous change of the magnification. In general, therefore, the degrees of magnification have been non-continuously selected and stepwisely changed.
It is cumbersome, however, that the mirror for compensating the optical path length is moved upon the changing of the magnification. It is desirable to change the magnification merely by moving the lens. It is further desirable that the extremities of an image can be aligned with one another by trimming, that is, minutely changing the magnification in the copying machines. Thus, a proposal has been made that the continuous change of the magnification may be effected by using a zoom lens in the copying machines U.S. Pat. No. 4,037,937 and others disclose photoengraving zoom lenses. When these zoom lenses are assembled into the projecting systems in the copying machines, problems are brought about by difficult compaction, small angle of view and large F-number.
U.S. Pat. Nos. 3,687,522 and 4,061,419 disclose compact zoom lens systems of the so-called symmetric orthometer type in each of which one or two lens elements therein can be moved upon zooming. Either of such compact zoom lens systems includes only six lenses resulting in reduced degrees of freedom in compensating aberrations. Accordingly, there is a problem in that the aberrations cannot be correctly set upon changing the magnification to reduce the resolving power.
U.S. Pat. No. 3,728,010 discloses a zoom lens system in which positive lenses are located outside the symmetric orthometer type lens arrangement symmetrically with respect to a diaphragm to form a fixed group. In this zoom lens system, lenses other than the positive lenses disposed outside as the fixed group are moved symmetrically relative to the diaphragm without change of the length of the lens system along the optical axis. However, there are problems in that the movable lenses are shifted through too large a distance and that the entire positive power in the movable lens group tends to be summed into the positive power in the fixed group to increase the Petzval Sum.